the default camera is blue, thats camera 0, any other camera has a green backdrop so that you can tell that you are not looking at the default or main camera, once you make a level you will not be able to see the backdrop anyway, or if you can then enclose the whole thing inside a giant sphere the colour you want, but make sure it isn`t that big that it is out of range of the camera and set the object cull to 1 or you will not be able to see it from the inside.
why do they say Aliens are gonna be friendly?, surely the agressive ones wipe out the peaceloving ones, so all you have left out there are crazed flesh eating interstellar monsters.